Animal health authorities have been investigating the novel H1N1 influenza virus, which sickened thousands of people worldwide, to determine what impact it could have on animals.

The virus strain, initially labeled by federal health authorities as swine influenza, had not infected any swine in the United States by early May.

"A lot of diagnostic samples have gone to the veterinary diagnostic labs throughout the influenza season, and there are labs that are going back and looking at those viruses and sequencing them to see if this novel H1N1 sequence is there," Dr. Harry O. Snelson, communications director for the American Association of Swine Veterinarians, said in late April. "To date, there have been none that have been positive for this gene sequence."

The Canadian Food Inspection Agency confirmed May 2 the H1N1 virus was found in pigs on an Alberta farm, and the pigs were likely exposed to the virus by a worker with flulike symptoms who had recently returned from Mexico. The herd was quarantined and the worker has recovered.

World Health Organization officials announced in late April the virus had caused sustained infections through human-to-human contact in communities in multiple countries. Before the end of the month, there were laboratory-confirmed cases in Austria, Canada, Germany, Israel, Mexico, New Zealand, Spain, the United Kingdom, and the United States.

Dr. Snelson said the Department of Agriculture's National Veterinary Services Laboratories and National Animal Disease Center were considering development of a test to differentiate between the novel H1N1 and typical influenza viruses that circulate in swineherds. He also said officials with those agencies would perform challenge studies to determine the pathogenesis and transmissibility of the virus, the effectiveness of vaccines, and the clinical signs if swine become infected.

Although it is not feasible to test all hogs for the virus, Dr. Snelson said in late April, "The chances are pretty slim that it's circulating in the U.S. swineherd."

The USDA, through a program started in the fall, has been collaborating with human health authorities in collecting information on the prevalence of swine influenza virus strains in humans and animals.

The USDA Agricultural Research Service launched in September 2008 a two-year pilot swine influenza surveillance program intended to provide a centralized monitoring system and help protect human and animal health, officials with the USDA-ARS previously said. The project was started in collaboration with the Centers for Disease Control and Prevention.

The pilot program could result in a permanent USDA-funded swine influenza surveillance program that would include sharing of information on infection among the agencies.

The influenza strain identified in the outbreak among humans contains genetic pieces of swine-, avian-, and human-type influenza viruses, according to the Centers for Disease Control and Prevention. The virus is believed to have spread among people who had no contact with pigs.

Federal authorities, swine industry representatives, and veterinary organizations urged pork producers to take precautions, however, to ensure their animals do not become infected.

David Warner, communications director for the National Pork Producers Council, said his organization encourages producers to vaccinate pigs and workers against influenza viruses.

In addition to addressing consumer concerns over pork, which the USDA and CDC have said is safe, Warner said his organization has been responding to comments from groups opposed to animal agriculture that have blamed the outbreak on concentrated animal feeding operations. He said swine held in such production facilities are less likely than those in open-air operations to have contact with wildlife, including birds that could carry avian flu.

Dr. Snelson said the AASV has been asking pork producers to be aware of the outbreak in humans. Because people are carrying a virus that is not in pigs—and because humans can transmit influenza to pigs—Dr. Snelson said it is important to keep the newly discovered H1N1 influenza strain out of farms.

"From what we understand, the pigs are at more risk from people than people are from the pigs," Dr. Snelson said.

The AASV is urging producers to take biosecurity precautions, limit visits to farms, and allow sick employees to stay home, Dr. Snelson said.

Dr. W. Ron DeHaven, AVMA CEO, said the AVMA has been in contact with the AASV, whose members are aware of the situation and are working with state and federal authorities to increase surveillance for the virus.

Cynthia Cunningham, assistant vice president of communications for the National Pork Board, said the CDC and USDA have been helpful in telling consumers that pork is safe to eat, and the pork board has also been trying to assure consumers that pork is safe.

The USDA has recommended that commercial pork producers not loan equipment or vehicles to other farms and avoid bringing swine from outside sources to their farms. The department also encouraged allowing only essential workers and vehicles on farms and ensuring that workers disinfect their shoes, clothes, and hands as well as vehicles and equipment entering or leaving the farm.